Question: In the right triangle shown, $\angle A = 30^\circ$ and $BC = 5$. How long is $AB$ ? $A$ $C$ $B$ $5$ $x$
Answer: We know the length of a leg, and want to find the length of the hypotenuse. What mathematical relationship is there between a right triangle's legs and its hypotenuse? We can use either sine (opposite leg over hypotenuse) or cosine (adjacent leg over hypotenuse). This is a 30-60-90 triangle, so we know what the values of sine and cosine are at each angle of the triangle. Let's try using sine: $A$ $C$ $B$ $5$ $x$ ${30}^{\circ}$ Sine is opposite over hypotenuse (SOH CAH TOA), so $\sin {30}^{\circ} = \dfrac{5}{x}$ . We also know that $\sin{30}^{\circ} = \dfrac{1}{2}$ Solving for $x$ , we get $ x \cdot \sin{30}^{\circ} = 5$ $ x \cdot \dfrac{1}{2} = 5$ $ x = 5 \cdot 2$ So, $x = 10$.